Providing high volume sewage treatment, as well as treatment of the sludge produced, is a challenge in many countries. Sludge disposal amounts up to 50% of sewage treatment cost, requires energy consumption and produces greenhouse gas emissions. Processes eliminating the need for sludge disposal without the need for a major sulfur source are environmentally and financially desirable.
The conventional biological carbon and nitrogen removal process has changed very little since the discovery of an activated sludge process in 1914 by Ardern and Lockett. The biological processes involved in secondary treatment have remained the same for almost a century, i.e. with electron flow from carbon to oxygen through heterotrophic carbon oxidation. Introduction of the biological nitrogen removal (BNR) processes in the 1960s modified secondary treatment processes by introducing autotrophic nitrification and heterotrophic denitrification processes. In the conventional BNR process, the electron flows from organic carbon to oxygen through integrated carbon and nitrogen cycles as shown in FIG. 4. However, heterotrophic denitrification has a high sludge yield factor, producing excess sludge wastage. Therefore, the BNR process requires increased handling and disposal. Depending on the sludge age, about 50-60% of the organic carbon in the sewage will be converted to CO2, and the remaining 40-50% converted to sewage sludge.
In Hong Kong, saltwater is used for toilet flushing. By making use of sulfate ions available in the saline sewage, the Hong Kong University of Science and Technology developed the sulfate reduction autotrophic denitrification and nitrification integrated (SANI) process as shown in FIG. 5 (Lau et al., 2006; Lu et al., 2009; Wang et al., 2009). In the SANI process, sulfate oxidizes organic carbon into CO2 while sulfate reduces to dissolved sulfide by the sulfate reduction bacteria in the first reactor. Ammonia nitrogen oxidizes by oxygen into nitrate in the third reactor by autotrophic nitrifiers. The nitrate is then recycled to the second reactor reacting with sulfide ions to convert into nitrogen gas by the autotrophic denitrifiers while the sulfide is converted back to sulfate ions.
In a previous study, an internal sulfur cycling (ISC) process had been proposed to recycle the use of elemental sulfur, as shown in FIG. 1. Elemental sulfur was used as an electron acceptor and final product in the wastewater treatment, thus sulfate was not involved in this process. However, the ISC process did not take nitrogen removal into account. Sulfide oxidation with oxygen is difficult to control with the main product as elemental sulfur. This means that a large proportion of sulfur is further oxidized into sulfate and then removed from the system, thus resulting in sulfur loss.